Curved two-dimensional nanocomposites for battery electrodes

ABSTRACT

A battery electrode composition is provided that comprises a composite material comprising one or more nanocomposites. The nanocomposites may each comprise a planar substrate backbone having a curved geometrical structure, and an active material forming a continuous or substantially continuous film at least partially encasing the substrate backbone. To form an electrode from the electrode composition, a plurality of electrically-interconnected nanocomposites of this type may be aggregated into one or more three-dimensional agglomerations, such as substantially spherical or ellipsoidal granules.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present Application for Patent is a Continuation of U.S. patentapplication Ser. No. 13/436,766 entitled “Curved Two-DimensionalNanocomposites for Battery Electrodes” filed on Mar. 30, 2012, whichclaims priority to Provisional Application No. 61/470,781 entitled“Three-Dimensional Porous Particles Composed of Curved Two-DimensionalNano-Sized Layers for Li-ion Batteries” filed on Apr. 1, 2011, each ofwhich is expressly incorporated by reference herein.

BACKGROUND Field

The present disclosure relates generally to energy storage devices, andmore particularly to lithium-ion battery technology and the like.

Background

Owing in part to their relatively high energy densities, light weight,and potential for long lifetimes, lithium-ion (Li-ion) batteries areused extensively in consumer electronics. In many applications, Li-ionbatteries have essentially replaced nickel-cadmium andnickel-metal-hydride batteries. Despite their increasing commercialprevalence, further development of Li-ion batteries is needed,particularly for potential applications in low- or zero-emissionhybrid-electrical or fully-electrical vehicles, consumer electronics,energy-efficient cargo ships and locomotives, aerospace, and powergrids. Such high-power applications will require electrodes with higherspecific capacities than those used in currently-existing Li-ionbatteries.

Currently, carbon-based materials (e.g., graphite) are employed as thepredominant anode material in Li-ion batteries. Carbon (C), in the formof graphite, has a maximum or theoretical specific capacity of about 372milli-Ampere hours per gram (mAh/g), but suffers from significantirreversible capacity losses during the first formation cycling.Notably, during the first charge cycle, the battery electrolytedecomposes on the graphite surface and a significant number (typicallyin the range of about 5%-15%) of lithium ions present in the cathodeduring the initial charge (or intercalated into the graphite duringcharging) become buried in the decomposed layer of electrolyte andcannot be extracted therefrom upon discharge of the battery. The higherspecific surface area of other low voltage (e.g., about 0 V-1.2 V vs.Li/Li+) anode materials generally enhances the degree of electrolytedecomposition and, hence, the magnitude of the irreversible capacitylosses.

Capacity storage in a Li-ion battery anode may be improved to a degreeby substituting graphene for graphite to increase the number oflithiation sites (i.e., sites for the storage of lithium ions duringcharging). However, graphene-based anodes are susceptible to otherproblems, including still very high irreversible capacity losses uponinitial cycling (due to their high specific surface area, on which theelectrolyte decomposes), correspondingly low Coulombic Efficiency (CE)during both the first and also subsequent cycling, and limited overallstability caused by the separation of graphene layers. While compositeelectrodes employing graphene nanosheets, carbon nanotubes, and/orfullerenes have been shown to increase capacity, they have failed toprovide the level of stability required for widespread adoption inindustry. Graphene therefore remains unsuitable as a replacement tocurrent graphite-based anode technology.

A variety of higher capacity materials have been investigated toovercome the drawbacks of carbon-based materials. Silicon-basedmaterials, for example, have received great attention as anodecandidates because they exhibit specific capacities that are an order ofmagnitude greater than that of conventional graphite. Silicon has thehighest theoretical specific capacity among metals, topping out at about4200 mAh/g. Unfortunately, silicon and similar materials suffer fromtheir own significant challenges.

One limitation of silicon is its relatively low electrical conductivity.Another limitation is the relatively low diffusion coefficient oflithium in silicon, leading to a low conductivity of lithium ionspermeating silicon. The primary challenge of silicon-based anodematerials, however, is the volume expansion and contraction that occursas a result of lithium ion alloying or dealloying, respectively, duringcharge cycling of the battery. In some cases, a silicon-based anode canexhibit an increase and subsequent decrease in volume of up to about400%. The high level of strain experienced by the anode material cancause irreversible mechanical damage to the anode. Ultimately, this canlead to a loss of contact between the anode and underlying currentcollector.

To mitigate such detrimental effects, various composite electrodesformed from higher capacity materials and different carbon arrangementshave been explored, ranging from so-called three-dimensional (3-D)micron-sized particle structures, to one-dimensional (1-D) nanotubestructures, to zero-dimensional (0-D) nanoparticle structures. Thedifferent structures have been fabricated by a corresponding variety oftechniques, including physical mixing, decomposition of C- andSi-containing precursors, and other solution-based methods. However,each of these conventional structures has failed to provide the level ofperformance required for widespread adoption. Many suffer from limitedporosity available for volume changes in the active material,non-uniform material properties at the nanoscale, a high surface areathat leads to very large irreversible capacity losses and low CE uponinitial cycling as well as during subsequent cycling, and otherproblems.

Many high capacity cathode materials for Li-ion batteries also sufferfrom low electrical conductivity, a low diffusion coefficient oflithium, high volume changes during battery operation, or a combinationof such shortcomings.

Accordingly, despite the advancements made in electrode materials, highcapacity Li-ion batteries remain somewhat limited in their applicationsand there remains a need for improved electrodes for use in Li-ionbatteries. Improved anodes and cathodes and, ultimately, the improvedLi-ion batteries, could open up new applications and advance theadoption of so-called high-power devices such as those discussed above.

SUMMARY

Embodiments disclosed herein address the above stated needs by providingimproved battery components, improved batteries made therefrom, andmethods of making and using the same.

In some embodiments, a battery electrode composition is provided thatcomprises a composite material comprising one or more nanocomposites.The nanocomposites may each comprise a planar substrate backbone havinga curved geometrical structure, and an active material forming acontinuous or substantially continuous film at least partially encasingthe substrate backbone. In some designs, the one or more nanocompositesmay comprise a plurality of electrically-interconnected nanocompositesaggregated into a three-dimensional agglomeration, such as asubstantially spherical or ellipsoidal granule, for example. Thethree-dimensional agglomeration may comprise internal pores, which maybe left vacant to accommodate volume changes in the active material, orat least partially filled with other materials.

In other embodiments, a method is provided for manufacturing a batteryelectrode composition. Such a method may include providing one or moreplanar substrate backbones having a curved geometrical structure, and atleast partially encasing each substrate backbone with a continuous orsubstantially continuous active material film to form one or morecomposite material nanocomposites. The method may also includeaggregating the nanocomposites into a three-dimensional,electrically-interconnected agglomeration, and, in some instances, atleast partially filling pores between the nanocomposites in theagglomeration with other materials.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are presented to aid in the description ofembodiments of the invention and are provided solely for illustration ofthe embodiments and not limitation thereof.

FIGS. 1A-1B are cut-away views that illustrate example nanocompositestructures according to various embodiments.

FIG. 2 is a graphical flow diagram depicting the formation of an examplesilicon-based nanocomposite structure using a graphene substratebackbone according to a particular embodiment.

FIGS. 3A-3B illustrate example 3-D agglomerate structures according tovarious embodiments.

FIGS. 4A-4D illustrate several example curved planar nanocompositeshapes according to various embodiments.

FIG. 5 illustrates an example nanocomposite and protective coatingarrangement according to one or more embodiments.

FIG. 6 is a cross-sectional view illustrating the lithiation anddelithiation of an example shell structure protective coatingarrangement according to one or more embodiments.

FIGS. 7A-7F are cross-sectional views illustrating several differentexample composite protective coating structures according to variousembodiments.

FIGS. 8A-8C are cross-sectional views illustrating the effects oflithiation and delithiation on the example composite protective coatingstructures of FIGS. 7D-7F.

FIG. 9 is a graphical flow diagram from a cross-sectional perspectivedepicting a formation of the example composite protective coatingstructures of FIGS. 7A-7F according to one or more embodiments.

FIG. 10 is a process flow diagram illustrating an example method offorming a curved planar nanocomposite according to one or moreembodiments.

FIGS. 11A-11F present several scanning electron microscopy (SEM)micrographs (FIGS. 11A, 11C, 11E, 11F) and transmission electronmicroscopy (TEM) micrographs (FIGS. 11B, 11D) showing the formation ofone particular example Si-graphene nanocomposite of the type illustratedin FIG. 2.

FIGS. 12A-12C illustrate several example characterizations of thegraphene and graphene-based nanocomposite shown in FIGS. 11A-11F byRaman spectroscopy (FIG. 12A), X-ray diffraction (FIG. 12B), and N2physisorption (FIG. 12C).

FIGS. 13A-13C illustrate example electrochemical performance data forsuch a C—Si-graphene electrode. FIG. 13A shows sample capacity data (permass of the composite) and Coulombic Efficiency (CE) data as a functionof cycle number, FIG. 13B shows a sample differential capacity plot, andFIG. 13C shows sample charge-discharge profiles for selected cycles.

FIG. 14 illustrates an example Li-ion battery in which the abovedevices, methods, and other techniques, or combinations thereof, may beapplied according to various embodiments.

DETAILED DESCRIPTION

Aspects of the present invention are disclosed in the followingdescription and related drawings directed to specific embodiments of theinvention. The term “embodiments of the invention” does not require thatall embodiments of the invention include the discussed feature,advantage, process, or mode of operation, and alternate embodiments maybe devised without departing from the scope of the invention.Additionally, well-known elements of the invention may not be describedin detail or may be omitted so as not to obscure other, more relevantdetails.

As discussed in the background above, despite the potential forcomposite electrodes to improve the performance of batteries fabricatedfrom higher capacity materials, conventional composite electrodes havethus far failed to provide adequate functionality. In particular,conventional 3-D micron-sized particle structures tend to exhibitrelatively poor ionic conductivity due to their large diffusiondistances. Conversely, smaller 0-D nanoparticle structures that providethe fast diffusion needed for high power characteristics and quickcharging times tend to suffer from low electrical conductivity and highthermal resistance.

Accordingly, improved nanocomposite material compositions and associatedelectrodes are provided herein that employ a substantiallytwo-dimensional (2-D) yet curved planar geometry. The planar designprovides decreased surface area for a given amount of active material,while maintaining a suitable ion diffusion distance, and the curvedgeometry ensures sufficient porosity of the electrode. Each of theseadvantages helps mitigate the low electrical and ionic conductivitycharacteristic of many higher capacity materials.

FIG. 1A is a cut-away view that illustrates an example nanocompositestructure according to one or more embodiments. As shown, ananocomposite 100 is formed from a planar substrate backbone 110structured with a curved morphology and an active material 120 forming acontinuous or substantially continuous film at least partially encasingthe substrate backbone 110. Although a fully continuous film that whollyencases the substrate backbone 110 may be desirable, it will beappreciated that the active material film 120 need not entirely coverthe substrate backbone 110 to be operational. Substantial coverage maybe sufficient in some designs.

The nanocomposite 100 is planar in the sense that it is generally formedin the 2-D shape of a sheet with a relatively large width-to-thicknessaspect ratio, but curved in the sense that, rather than being flat, thesheet is characterized by a non-zero radius of curvature in at leastsome areas. For example, in some designs, it has been found that theplanar morphology of the nanocomposite 100 may be characterized by awidth-to-thickness aspect ratio for the substrate backbone 110 in therange of about 10 to about 1,000,000, with an average thickness of thesubstrate backbone 110 in the range of about 0.2 nm to about 100 nm. Inaddition, in some designs, it has been found that the curved morphologyof the substrate backbone 110 may be characterized by a radius ofcurvature in the range of about 0.3 nm to about 0.03 mm.

The curved planar geometry reduces the surface area of the nanocomposite100 for a given active material mass and a given maximum diffusiondistance, as compared to conventional designs. A low surface area isimportant for reducing exposure of the active material 120 to thebattery electrolyte, and hence, for reducing degradation of theelectrode. In particular, a low surface area leads to a correspondinglysmaller total mass and volume of the solid-electrolyte interphase (SEI),which often forms upon electrolyte decomposition (on initial charging)at the boundary between the active material and the electrolyte mixturein each cell. The formation of the SEI acts as a barrier to furtherelectrolyte decomposition. This decomposition occurs via electrochemicalreactions (e.g., commonly reduction of the electrolyte on the anode oroxidation of the electrolyte on the cathode of a battery, such as aLi-ion battery) at the interface between the active material and certainsolvent molecules present in the electrolyte. If the SEI fails tosufficiently suppress such electrolyte decomposition during batteryoperation, rapid battery degradation may occur.

The SEI is routinely subjected to various chemical as well as physicaldegradation processes throughout a cell's operation, however, which maycause it to become either electrically conductive or substantiallypermeable to solvent molecules. This, in turn, causes furtherelectrolyte decomposition and SEI growth, significantly decreasingbattery lifetime as well as energy and power characteristics. Decreasingthe overall volume of the SEI reduces the total irreversible capacitylosses. Decreasing the surface area of the SEI (for a fixed thickness)not only decreases the total SEI volume within the electrode, but alsodecreases the probability that a void or other defect will occur overthe SEI surface that would allow electrolyte solvent molecules to comeinto contact with the active material at an undesirably fast rate.Decreasing the SEI surface area further decreases the average flux ofsolvent molecules slowly permeating through the SEI, which causes theeventual decomposition irreversible losses. In practice, the diffusionof solvent molecules through the SEI cannot be infinitely slow, but itis desirable for this diffusion process to be as slow as possible orpractical. Finally, decreasing the total area of the active materialexposed to the electrolyte decreases the rate of any other unwantedchemical reactions.

The curved planar geometry of the nanocomposite 100 described hereinprovides a smaller SEI for a given active material mass as compared toconventional designs. Additionally, the curved planar geometry may leadto enhanced SEI stability and improved performance as a barrier for theunwanted reactions and diffusion of electrolyte solvent molecules.

At the same time, the curved planar geometry also maintains a suitablysmall diffusion distance for a given active material mass. The diffusiondistance over which active ions (e.g., Li ions in a Li-ion battery) musttravel during insertion into and extraction from the active materialupon charge cycling of the battery is an important factor in providingquick charging times and suitable power characteristics. If the activeions have to travel too far into or out of the active material, chargingand discharging times will suffer.

In some embodiments, for active materials with low electricalconductivities, the active material 120 may be deposited on anelectrically conductive material used as the substrate backbone 110 tominimize the resistance for electrons moving to (or from) the desiredelectrochemical reaction site from (or to) the electrically conductivebackbone 110 during battery charging or discharging. In otherembodiments, if the substrate backbone 110 is not electricallyconductive and the active material 120 also does not have highelectrical conductivity, an additional coating of an electricallyconductive and ionically permeable layer may be provided on the surfaceof the active material 120.

FIG. 1B is a cut-away view that illustrates an example nanocompositestructure having an additional coating of this type according to one ormore embodiments. As in FIG. 1A, the nanocomposite 150 in the design ofFIG. 1B is formed from a planar substrate backbone 110 structured with acurved morphology and an active material 120 forming a continuous orsubstantially continuous film at least partially encasing the substratebackbone 110. However, the nanocomposite 150 additionally includes anouter coating 130 at least partially covering the surface of the activematerial 120. Carbon is one example of a material that may be used forsuch a coating, but other electrically conductive and ionicallypermeable materials may also be used. In this case, duringelectrochemical reaction (charge or discharge of a battery), both ionsand electrons permeate toward the electrochemical reaction sites withinthe active material 120 through the surface of the outer coating 130.The curved planar geometry in this design similarly reduces thediffusion distance for both ions and electrons for a given externalsurface area of the active material 120, while the outer coating 130helps improve electrical conductivity.

Conventionally, a small diffusion distance has been achieved byconstructing so-called 0-D nanoparticles, as discussed above. However,nanoparticles tend to suffer from low electrical conductivity due toincreased particle-to-particle resistances, which causes generation ofheat during high current pulses within the battery, and, perhaps moreimportantly from a safety perspective, nanoparticles tend to suffer fromhigh thermal resistance because of the high phonon scattering takingplace at the point contacts between nanoparticles. By contrast, thecurved planar geometry of the nanocomposites described herein is able toprovide a suitably small diffusion distance for a given active materialmass by spreading the active material out in a thin film over thesubstrate backbone, as well as provide significantly better electricalconductivity and thermal transport properties. This ensures that more ofthe active material remains close to the structure's surface, withoutencountering the drawbacks associated with the small size ofconventional nanoparticles.

Again though, this is achieved without dramatic increases in surfacearea, which leads to the degradation problems discussed above. Otherconventional approaches such as 1-D nanowire structures, for example,have a much higher surface area for the same diffusion distance, andsuffer from other problems including gaps that form between thenanowires.

The curved planar geometry is also often able to better accommodatevolume changes during battery operation. This is important for somehigher capacity active materials such as silicon, which experiencesignificant volume fluctuations (up to 400% for some materials) duringinsertion and extraction of active ions, such as lithium (Li) in aLi-ion battery. SEI stability in particular, and its resistance againstpermeation of electrolyte solvent molecules and other harmfulcomponents, can be compromised by the large volume changes in siliconduring lithium insertion/extraction, as the outer surface area iscontinually expanded and contracted. This expansion/contractiontypically causes the formation of SEI defects and voids, which lead tothe degradation of its resistance to electrolyte solvent permeation andthus to irreversible capacity losses, SEI growth, and the resultantdecrease in both energy and power characteristics of the battery. Thelow elasticity of the SEI makes it difficult to achieve long-termstability under cycling load for designs that strongly correlate changesin volume with changes in outer surface area.

Conventional 0-D, 1-D, and 3-D composites strongly correlate changes involume with changes in outer surface area, and are accordinglysusceptible to the type of SEI defects and voids described above.Silicon nanoparticles and nanowires, for example, experience very largeexternal surface area changes during insertion/extraction of lithium.Silicon nanoparticles in particular expand uniformly in all dimensions,and thus their outer surface area (where the SEI forms) changesdramatically during insertion/extraction of lithium.

By contrast, the inventors of the present application have shown thatthe curved planar geometry of the nanocomposites described herein isable to accommodate volume swelling primarily through changes inthickness. This is advantageous because changes in thickness arerelatively decoupled from changes in outer surface area. Because theouter surface area is able to remain substantially constant during ioninsertion/extraction, the nanocomposites described herein are moreeasily able to maintain stability of the SEI, ensuring that it remainsimpermeable for solvent molecules, becomes electrically insulating,maintains good adhesion, and so forth.

Returning to FIGS. 1A and 1B, the substrate backbone 110 providing thefoundation for the curved planar geometry may be formed from a varietyof materials. In general, any curved, thin 2-D substrate that does notreact with precursors of the active material 120 may be used.

One example material for the substrate backbone 110 is graphene.Graphene is an allotrope of carbon and may be structured in one or moreatom-thick planar sheets or layers of sp²-bonded carbon atoms denselypacked in a honeycomb crystal lattice. The high surface area of graphenerelative to its size (i.e., its specific surface area) allows rapidactive material deposition, which may be advantageous for certainpractical applications and manufacturing considerations.

FIG. 2 is a graphical flow diagram depicting the formation of an examplesilicon-based nanocomposite structure using a graphene substratebackbone according to a particular embodiment. In this example,formation begins with the exfoliation and shearing of a natural graphitestarting material or the like (block 210) to produce graphene (block220). Mechanical shearing, for example, may be used to produce grapheneof very high quality. For better process scalability, however,ultrasonic shearing may alternatively be used for graphene productionwith a significantly higher yield. Other exfoliation and shearingprocedures as known the art may also be used.

In one particular example, a natural graphite powder may be initiallyimmersed in concentrated sulfuric acid for a given duration (e.g., abouttwo hours has been found to be sufficient). The subsequently driedproduct may then be subjected to thermal shock treatment at elevatedtemperature (e.g., about 1,000° C.) for a given duration (e.g., about 30seconds) to obtain exfoliated graphite worms. The exfoliated graphiteworms may then be dispersed in deionized water and subjected toextensive ultrasonication treatment to break down the graphite flakesand separate the graphene sheets. The dried product, graphene oxide, maybe reduced at elevated temperature (e.g., about 800° C.) in a gaseousstream such as H₂ for about one hour to obtain a graphene powder.

Returning to FIG. 2, the produced graphene may then be uniformly coatedwith the desired active material (block 230). For the silicon activematerial in this example, the coating may be achieved via silane (SiH₄)decomposition at elevated temperature or in other ways as desired. Forexample, low pressure decomposition of a high purity SiH₄ at 500° C. maybe utilized to deposit silicon onto graphene. As discussed above, incontrast to nanoparticles, the thicker nanocomposite structures that maybe produced in this manner provide a lower surface area for a givenmass, and better potential for reducing the irreversible capacity lossesexperienced upon the first and subsequent cycles of the resultantbattery.

Other finishing steps may be performed as desired for particularapplications. Here, a thin layer of amorphous carbon is deposited (block240) similar to the design of FIG. 1B, by way of example, to reducesilicon oxidation, improve anode stability, and increase anodeelectrical conductivity and battery power characteristics. The carboncoating may be deposited, for example, via atmospheric pressuredecomposition of a carbon precursor (e.g., C₃H₆) at elevated temperature(e.g., at about 600-1000° C.) for a given duration (e.g., for about 5-50minutes). In some processes, it may be desirable to place a mineral oilbubbler in line with the gas exhaust line to avoid oxidation caused bythe back-stream diffusion of oxygen or oxygen-containing molecules fromthe exhaust side. In this way, a composite containing about 60 wt. %silicon may be produced. Silicon and carbon contents may be calculatedvia mass change measurements, for example, after correspondingdepositions, and verified using thermogravimetric analysis or equivalentprocedures. Particular example fabrications and their characterizationsare described later.

One of the advantages of using graphene as the substrate backbone isthat it may be formed as a single layer or as multiple layers. Further,according to various embodiments, the thickness and distance between thegraphene layers may be tuned to adjust the thickness of the depositedactive material film. Thicker films of active material generally providehigher capacity but slower charge transfer rates, while thinner filmsgenerally provide more rapid charging and higher power performance. Inthis way, the tunable spacing between individual graphene layersprovides for active material film depositions of various thicknesses,and thus, optimization of the nanocomposite for either high energy orhigh power applications.

Other materials may also be used to form the substrate backbone 110providing the foundation for the curved planar geometry. Examples ofsuch curved layered or planar materials include but are not limited tovarious clay minerals (e.g., Al(OH)₃, Mg(OH)₂), exfoliated boron nitride(BN), exfoliated molybdenum disulfide (MoS₂), exfoliated tungstendisulfide (WS₂), bismuth telluride (Bi₂Te₃), exfoliated layered ternarycarbides (e.g., Ti₂SiC₂), and others.

A variety of active materials may be advantageously structured in thecurved planar geometry nanocomposites described herein, including notonly silicon (Si) discussed above and similar materials that experiencesignificant volume changes (e.g., greater than about 5%), but also othermaterials where volume change accommodation is not of concern. Thecurved planar geometry of the nanocomposites described herein isbeneficial and provides good electrical and thermal conductivity for awide range of active cathode and anode materials for Li-ion and otherbatteries.

Among anodes, examples of such materials include: (i) heavily (and“ultra-heavily”) doped silicon; (ii) group IV elements; (iii) binarysilicon alloys (or mixtures) with metals; (iv) ternary silicon alloys(or mixtures) with metals; and (v) other metals and metal alloys thatform alloys with lithium.

Heavily and ultra-heavily doped silicon include silicon doped with ahigh content of Group III elements, such as boron (B), aluminum (Al),gallium (Ga), indium (In), or thallium (Tl), or a high content of GroupV elements, such as nitrogen (N), phosphorus (P), arsenic (As), antimony(Sb), or bismuth (Bi). By “heavily doped” and “ultra-heavily doped,” itwill be understood that the content of doping atoms is typically in therange of about 3,000 parts per million (ppm) to about 700,000 ppm, orapproximately 0.3% to 70% of the total composition.

Group IV elements used to form higher capacity anode materials mayinclude Ge, Sn, Pb, and their alloys, mixtures, or composites, with thegeneral formula of Si_(a)—Ge_(b)—Sn_(c)—Pb_(d)—C_(e)-D_(f), where a, b,c, d, e, and f may be zero or non-zero, and where D is a dopant selectedfrom Group III or Group V of the periodic table.

For binary silicon alloys (or mixtures) with metals, the silicon contentmay be in the range of approximately 20% to 99.7%. Examples of such asalloys (or mixtures) include, but are not limited to: Mg—Si, Ca—Si,Sc—Si, Ti—Si, V—Si, Cr—Si, Mn—Si, Fe—Si, Co—Si, Ni—Si, Cu—Si, Zn—Si,Sr—Si, Y—Si, Zr,—Si, Nb—Si, Mo—Si, Tc—Si, Ru—Si, Rh—Si, Pd—Si, Ag—Si,Cd—Si, Ba—Si, Hf—Si, Ta—Si, and W—Si. Such binary alloys may be doped(or heavily doped) with Group III and Group V elements. Alternatively,other Group IV elements may be used instead of silicon to form similaralloys or mixtures with metals. A combination of various Group IVelements may also used to form such alloys or mixtures with metals.

For ternary silicon alloys (or mixtures) with metals, the siliconcontent may also be in the range of approximately 20% to 99.7%. Suchternary alloys may be doped (or heavily doped) with Group III and GroupV elements. Other Group IV elements may also be used instead of siliconto form such alloys or mixtures with metals. Alternatively, other GroupIV elements may be used instead of silicon to form similar alloys ormixtures with metals. A combination of various Group IV elements mayalso be used to form such alloys or mixtures with metals.

Examples of other metals and metal alloys that form alloys with lithiuminclude, but are not limited to, Mg, Al, Ga, In, Ag, Zn, Cd, etc., aswell as various combinations formed from these metals, their oxides,etc.

Examples of cathode active materials that may benefit from the curvedplanar geometry nanocomposites described herein include: variousfluorides, various iodides, various oxides, various oxy-halides, variousphosphates, various sulfides, sulfur, iodine and various othersulfur-containing compounds, among other materials, and their variousmixtures.

Turning now to the larger structure of the electrode itself, electrodesaccording to various embodiments herein may be formed from a pluralityof nanocomposite “building blocks” agglomerated or assembled together.That is, the curved planar geometry nanocomposites described above maybe assembled into 3-D agglomerates of various forms that provide goodthermal transfer properties as well as good electrical conductivity. Forexample, the agglomerates can be assembled in the form of a bulk unitarybody that adopts the ultimate shape of the electrode. Alternatively, theagglomerates can be assembled in the form of a specially shaped granule,which can be packed together in close proximity with other such granulesto form the electrode. A polymer binder may be used to hold thesegranules together and attach them to the surface of a current collector.

FIG. 3A illustrates an example 3-D agglomerate structure according to aparticular embodiment. In this example, the 3-D agglomerate structure300 includes a plurality of nanocomposites 302 with pores or voids 304therebetween, and is simply assembled in the form of a bulk unitary bodythat adopts the shape of the underlying current collector 310 of theelectrode.

FIG. 3B illustrates an example 3-D agglomerate structure according toanother embodiment. In this example, the 3-D agglomerate structure 350also includes a plurality of nanocomposites 302 with pores or voids 304therebetween, but is shaped as a generally spherical granule. It will beappreciated, however, that other shapes may be created as desired for aparticular application, such as an ellipse, an ellipsoid, a rod, orother shapes. It will further be appreciated that while terms like“spherical” and “ellipsoidal,” for example, are used to describe theshape of such a 3-D structure, these terms are not intended to convey orin any way require that the agglomerates take on a mathematicallyprecise geometric figure. These terms are only used to convey thegeneral shape for illustration purposes. It will be appreciated that, inpractice, the 3-D agglomerate structure 350, for example, may simply besubstantially round, and not precisely spherical or even ellipsoidal.

Each individual nanocomposite 302 within the agglomerates 300, 350 isassembled in such a manner as to be in electrical communication with atleast one other nanocomposite 302. In some designs, this may beaccomplished simply by forming the agglomeration in such a way that atleast an electrically conductive portion of each nanocomposite 302 is incontact with at least an electrically conductive portion of anothernanocomposite 302. In other designs, an additional electrically orionically conductive coating may be applied to the surface of eachnanocomposite 302, such as via the outer coating 130 shown in FIG. 1B.For example, vapor deposited carbon or carbon produced by adecomposition of hydrocarbons or various organic compounds may be usedin various embodiments to electrically connect the individual buildingblock nanocomposites 302 within the agglomerates 300, 350.

Alternatively or in addition, an electrically conducting additive can beused to ensure that the different nanocomposites 302 are in sufficientelectrical contact with one another. One example of such an electricallyconductive additive is carbon, which can be produced from a sacrificialorganic binder that converts primarily to carbon during fabrication andprior to assembly of the electrode. Exemplary binders include polymericmaterials having at least about 20 atomic percent carbon in themonomeric unit used to form the polymer. Suitable polymers include butare not limited to sucrose, carbonates, petroleum pitch, olyethers,polyesters, polymers based on methyl methacrylate, polymers based onacrylonitrile, polymers based on vinylidene fluoride, and the like.Other electrically conductive additives for such purposes includeconductive polymers, and are known to those skilled in the art to whichthis disclosure pertains. In this manner, the electrical conductivity ofthe electrode is not unnecessarily hindered by the surface resistance atnanocomposite-nanocomposite boundaries. Similarly, when the electrodecomprises a plurality of agglomerated granules, the granules may bepacked or arranged into a matrix in such a manner as to have eachgranule in electrical communication with at least one other granule,and, in some designs, a further conductive additive may be addedtherebetween.

As discussed above, the individual nanocomposites in the agglomeratedstructures are themselves advantageously able to absorb volume changesassociated with certain active materials in an efficient manner withrespect to changes in surface area. Similarly, in some examples thecurved shape of the 2-D nanocomposites creates pores within the largeragglomerated structure, as shown, for example, in FIGS. 3A and 3B by thepores 304. The additional volume provided by the pores provides furtherspace for active material expansion and contraction during ion insertionand extraction, respectively. Accordingly, regardless of how thenanocomposites are assembled, the curved shape of each nanocompositeensures that the anode will maintain at least some level of porositybetween the layers of active material. The exact porosity of theelectrode will depend, at least in part, on the particular structure ofthe individual nanocomposites. As will be discussed in more detailbelow, these properties may be adjusted according to various embodimentsto achieve a desired porosity for a given application.

Generally though, the available pore volume can be made sufficientlylarge to accommodate active material expansion and contraction duringcharge cycling (e.g., in some cases up to about five times the volume ofthe active material in the agglomerated nanocomposites). This thresholdpore space prevents the electrode from experiencing strain as a resultof active material swelling. In order to minimize any adverse effects onthe volumetric performance of the electrode, it may be desired to limitthe pore volume to less than about 90% of the total volume of theagglomerate. It has been found that when the pore volume is greater thanabout 90% of the total geometrical volume of silicon nanocompositeaggregates, for example, the volumetric capacitance of the anode maybegin to suffer.

In some embodiments, the voids or pores between the layers of activematerial may be at least partially filled with an ionically conductivematerial, such as a solid electrolyte or an active material withsignificantly higher ionic conductivity (but suffering, for example,from high cost or low capacity or other limitations). In someembodiments, this pore-filling material may have mixed (electronic andionic) conductivity.

As discussed above, other types of active materials that do notexperience significant volume changes may also be advantageouslystructured in the form of the nanocomposites and the agglomerations ofnanocomposites described herein. For materials without significantexpansion or contraction, the porosity may be made small to increasevolumetric capacity. Alternatively, however, the pores between theagglomerated nanocomposites may be repurposed to enhance otherproperties of the resultant electrode. For example, similar to thepore-filling described above, the pores 304 shown in FIGS. 3A and 3B maybe completely or partially filled with an ion-conducting material suchas a solid electrolyte to enhance ionic conductivity.

In some embodiments, this solid electrolyte can be a polymerelectrolyte. Polymer electrolytes differ from the liquid electrolytescommonly used in Li-ion battery cells. Rather than holding alithium-salt electrolyte in a liquid organic solvent, polymerelectrolytes use a solid polymer composite such as polyethylene oxide orpolyacrylonitrile. Polymer electrolytes have garnered significantinterest as a replacement to conventional liquid electrolytes because oftheir potentially lower cost of manufacture, adaptability to a widevariety of packaging shapes, reliability, ruggedness, and, perhaps moreimportantly, enhanced safety. Polymer electrolytes may, in some designs,produce a more stable SEI than liquid electrolytes. However, despitethese potential advantages, they have proven impractical for a number ofreasons.

In particular, polymer electrolytes cannot be formed into thin layersand must be made substantially thicker than their liquid electrolytecounterparts, often an order of magnitude or more, which significantlyincreases the diffusion distance for active ions such as lithium.Diffusion through a polymer electrolyte is therefore often ten timesslower than diffusion through a liquid electrolyte in the same cell,and, in many practical applications, even slower. Although slowdiffusion may actually be a benefit to certain applications in whichsafety is a priority, it severely impedes the use of polymerelectrolytes for higher power applications.

The pores 304 created by the nanocomposite agglomerations 300, 350provide a substantially isolated region in the cell in which to employpolymer electrolytes without the conventional drawbacks typicallyassociated with their use. In this way, each type of electrolyte may beused in the area for which it is best suited, offering the best of bothworlds by providing a combination of polymer electrolytes within theactive electrode material agglomerates and liquid electrolytes elsewherethroughout the cell. Such a design provides fast ion transfer propertieswithin the electrode, fast electron transfer properties within theagglomerate, a diffusion distance through the active material thatremains substantially small, and so forth, leading to fast powercharacteristics while at the same time advantageously maintaining alarge effective particle size.

Charging and discharging of a battery (including a Li-ion battery)during its operation requires access of both electrolyte ions andelectrons to the reaction sites of the active material. Therefore,minimizing the resistance faced by both ions and electrons traveling toeach reaction site is particularly important for maintaining high powerperformance. Further, the surface area in such a design remainsmoderate, as discussed above, which is advantageous for battery safety,as a high surface area more easily leads to thermal runaway, which,combined with the poor thermal conductivity of conventional designs,stores heat inside the cell and may render it unsafe.

In other embodiments, the solid electrolyte that at least partiallyfills the pores 304 may be a ceramic-based electrolyte. Ceramic-basedelectrolytes are highly resistant to decomposing at the interface withthe active material, they provide very safe operation, and they providea relatively long cycle life. For reasons similar to those describedabove for a polymer electrolyte, it may be beneficial in certainembodiments for both cost and performance reasons to use a liquidelectrolyte between the agglomerates and a ceramic electrolyte withinthe agglomerates.

In still other embodiments, the pores 304 may be at least partiallyfilled with a hybrid of two types of materials employed together in thesame design—one with high electrical conductivity (e.g., on the order of10⁻⁸ S cm⁻¹ or better) and another with high ionic conductivity (e.g.,on the order of 10⁻⁸ S cm⁻¹ or better), for example. In some designs, itmay be advantageous to use a first ion-permeable and electricallyconductive material covering the surface of the active material layerand a second ionically conductive but electrically insulative materialcovering at least a portion of the surface of the electricallyconductive material. In some designs, it may be further advantageous tomake the second ionically conductive but electrically insulativematerial permeable only to electrolyte ions but have very small (if any)permeability to electrolyte solvent molecules. This type of design helpsto prevent or at least limit the solvent molecules from reaching thesurface of the active material and decomposing.

In still other embodiments, the pores 304 may be at least partiallyfilled with a material having a high mixed (electrical and ionic)conductivity. For example, in some designs, when the filling of pores isincomplete, it may be advantageous to make the second ionicallyconductive but electrically insulative material discussed above,covering at least a portion of the surface of the first material, have amixed conductivity. In some designs, it may be further advantageous tomake the second ionically conductive but electrically insulativematerial permeable only to electrolyte ions but have very small (if any)permeability to electrolyte solvent molecules. Again, this type ofdesign helps to prevent or at least limit the solvent molecules fromdecomposing.

As mentioned above, the density of the pores 304 created by thenanocomposite agglomerations 300, 350 may also be tailored according tovarious embodiments to trade off one or more of the performancecharacteristics of the cell, as desired for a particular application.For example, the pore density or pore size can be adjusted to reduceoverall pore volume, while still ensuring that pore volume (in somedesigns, filled with a liquid or solid electrolyte or with a highlyconductive material) is large enough for the ions to propagate at adesirable rate. This may be advantageous because most active materialsdo not have a high electrical or ionic conductivity.

One example mechanism by which the porosity can be adjusted is to changethe shape and associated curvature of the individual nanocompositesmaking up the agglomerations.

FIGS. 4A-4D illustrate several example curved planar nanocompositeshapes according to various embodiments. FIG. 4A illustrates aparticular example design in which the substrate backbone 110 is twistedto form a helix or spiral shaped nanocomposite 410. FIG. 4B illustratesanother particular example design in which the substrate backbone 110 isrolled up to form a different spiral shaped nanocomposite 420. FIG. 4Cillustrates another particular example design in which the substratebackbone 110 is curled to form a horseshoe shaped nanocomposite 430.FIG. 4D illustrates another particular example design in which thesubstrate backbone 110 is contorted to form a more arbitrarily shapednanocomposite 440. For simplicity, the outer coating 130 is omitted fromthe illustrations in FIGS. 4A-4D, but it will appreciated that one mayemployed as desired.

In any case, it has been found that designs in which the square root ofthe external surface area of the nanocomposite exceeds the nanocompositethicknesses by an order of magnitude or more provide good performance.It has also been found that designs in which the nanocomposite thicknessranges from about 0.003 μm to about 0.5 μm provide good performance.

In some embodiments, electrode stability may be further enhanced by theapplication of an additional protective coating to the surface of theactive material film. The protective coating may be formed from orotherwise include material specially selected to reduce or preventdegradation of the active material. Examples of such protective coatingmaterials may include select polymers, oxides, halides (such asfluorides or chlorides), oxyhalides (such as oxyfluorides oroxychlorides), carbon, sulfides, combinations thereof, or othermaterials. Such a coating should be permeable to ions inserted into orextracted from the active material, such as lithium ions in the case ofLi-ion batteries. Desirably, and in at least in some designs, theprotective coating is made resistant (if not impermeable) toinfiltration by the electrolyte solvents as well, which again are aleading contributor to the degradation of the electrolyte andirreversible capacity losses within the battery.

FIG. 5 illustrates an example nanocomposite and protective coatingarrangement according to one or more embodiments. In this example,similar to the example of FIG. 1B, the nanocomposite 500 includes aplanar substrate backbone 510, an active material layer 520, and (insome designs) an outer coating 530. A generic planar shape for thenanocomposite 500 is shown in FIG. 5 for illustration purposes, but itwill be appreciated that any suitable shape may be employed as desired,several examples of which are given above.

As shown, the nanocomposite 500 further includes a protective coating540 that at least partially encases the nanocomposite 500. As shown, theprotective coating 540 may be adhered onto the surface of thenanocomposite 500 and act as a barrier to the electrolyte solventscontained therein (while remaining permeable to, for example, lithiumions), preventing it from deteriorating the underlying active material520. As discussed above, the protective coating 540 may be formed fromselect polymers, oxides, fluorides, carbon, sulfides, combinationsthereof, or other materials.

In some embodiments, the protective coating is made flexible so that itcan generally expand and contract with the active material during ioninsertion and ion extraction. For example, the protective coating may bemade from a polymer chain that is able to slide over itself. Otherexamples include metals that naturally form protective oxide layers ontheir surfaces. These include, but are not limited to oxides of aluminum(Al), titanium (Ti), chromium (Cr), tantalum (Ta), niobium (Nb), andother metals or semimetals. Deposition of such coatings can be performedusing a variety of oxide coating deposition techniques, includingphysical vapor deposition, chemical vapor deposition, magnetronsputtering, atomic layer deposition, microwave-assisted deposition, wetchemistry, and others. Deposition of fluoride, sulfide, and other typesof protective coatings as well as solid electrolyte coatings on thesurface of the curved 2D active material can be produced using similartechniques.

As discussed above, wet chemistry methods can be employed to producemetal oxide protective coatings. For example, metal oxide precursors inthe form of a water-soluble salt may be added to the suspension (inwater) of the nanocomposites to be coated. The addition of the base(e.g., sodium hydroxide or amine) causes formation of a metal (Me)hydroxide. Active nanocomposites suspended in the mixture may then actas nucleation sites for Me-hydroxide precipitation. Once thenanocomposites are coated with a shell of Me-hydroxide, they can beannealed in order to convert the hydroxide shell into a correspondingoxide layer that is then well-adhered to the nanocomposite surface.

In other examples, an aluminum oxide coating may be produced using wetchemistry techniques according to the following or similar steps: (i)dissolving aluminum isopropoxide in ethanol; (ii) drying the solution inthe presence of active silicon particles; and (iii) annealing totransform the aluminum isopropoxide coating into an aluminum oxidecoating. In still other examples, an iron fluoride coating may beproduced using wet chemistry techniques according to the following orsimilar steps: (i) dissolving iron (Fe) powder in an aqueous solution offluorosilicic acid; (ii) drying the solution in the presence of activesilicon particles to form a coating composed of FeSiF₆.H₂O; and (iii)transforming the FeSiF₆.H₂O into iron fluoride (FeF_(x)) by annealingthe coated powder in an inert environment, such as argon gas or invacuum.

The formation of a sufficiently flexible protective coating may beadvantageous for certain active materials like silicon that exhibit anincrease, and subsequent decrease, in volume of up to about 300% duringcycling. Flexibility of the protective coating guards against theformation of cracks or voids that would otherwise allow harmfulparticles to penetrate through to the active material.

In other embodiments, however, it has been found that the use of amechanically stable and plastically deformable protective coating mayprovide further advantages when appropriately deployed. For example, ashell structure may be used to coat or encase the nanocomposite surface.During the initial insertion (e.g., the first cycle of a Li-ionbattery), the outer surface area of such a protective coating may beplastically deformed and expand to match the expanded state of thelithiated nanocomposite. However, upon lithium extraction, this type ofprotective coating remains mechanically stable and largely retains itsexpanded state. In this way, a shell structure is created that fits thenanocomposite in the expanded lithiated state, but allows forcontraction of the active material core during delithiation withoutfracturing.

FIG. 6 is a cross-sectional view illustrating the lithiation anddelithiation of an example shell structure protective coatingarrangement according to one or more embodiments. In this example, thenanocomposite 500 is designed as shown in FIG. 5, including a planarsubstrate backbone 510 and an active material 520, with the protectivecoating 540 being, in this case, a plastically deformable yetmechanically stable shell structure. For simplicity, the outer coating530 is omitted from the illustration in FIG. 6, but it will beappreciated that one may be employed as desired.

Here, the shell structure is designed such that, at least after aninitial cycling period, voids or pockets 612 are formed inside theprotective coating 540 (in the delithiated state) that provide space forthe active material 520 to expand during lithiation. In this way, theouter surface area of the protective coating 540 may remainsubstantially stable throughout the subsequent cycles of silicon-lithiumexpansion (during lithium insertion) and compaction (during lithiumextraction) as part of normal battery operation.

An SEI layer 630 is also shown in FIG. 6 to illustrate how it is formed(on initial charging) over the protective coating 540 rather thandirectly on the surface of the active material 520 as it otherwisewould. As discussed above, SEI stability and resistance againstpermeation of solvent molecules and other harmful components can becompromised by the large volume changes in certain active materialsduring lithium insertion/extraction, as its outer surface area iscontinually expanded and contracted, which typically causes theformation of SEI defects and voids, leading to the degradation of itsresistance to solvent molecule permeation. However, with the shellprotective coating in the design of FIG. 6 providing a more stable outersurface area for the SEI 630 to form on, the mechanical stability of theSEI layer 630 may be enhanced. A more stable SEI layer 630 providesadditional protections against various degradation processes.

In some embodiments, a ceramic coating material may be used to form theshell structure for the protective coating 540. Examples of ceramiccoatings include, but are not limited to: various sulfides, aluminumoxide or lithiated aluminum oxide, titanium oxide or lithiated titaniumoxide, zinc oxide or lithiated zinc oxide, niobium oxide or lithiatedniobium oxide, and tantalum oxide or lithiated tantalum oxide. Otherexamples of ceramic coatings include, but are not limited to: vanadiumfluoride, vanadium oxyfluoride, iron fluoride, iron oxyfluoride,aluminum fluoride, aluminum oxyfluoride, titanium fluoride, titaniumoxyfluoride, zinc fluoride, zinc oxyfluoride, niobium fluoride, niobiumoxyfluoride, tantalum fluoride, tantalum oxyfluoride, nickel fluoride,nickel oxyfluoride, magnesium fluoride, magnesium oxyfluoride, copperfluoride, copper oxyfluoride, manganese fluoride, and manganeseoxyfluoride. These ceramics are both Li-ion permeable and electrolytesolvent impermeable, making them well-suited for use in Li-ion batteryapplications, for example.

In other embodiments, a polymer coating may be used to form the shellstructure for the protective coating 540. For example, the polymercoating may be applied to the surface of the active material 520 as asacrificial layer and then carbonized (e.g., through pyrolysis orthermal annealing) to leave behind a carbon shell acting as theprotective coating 540.

In still other embodiments, a combination of materials may be used toform a composite protective coating. The appropriate selection andparticular application of different materials to form the compositecoating may lead to various further advantages, depending on the desiredapplication, as the composite may be designed to take advantage ofseveral disparate physical or chemical properties of the two or moreconstituent materials not found in either material alone.

FIGS. 7A-7F are cross-sectional views illustrating several differentexample composite protective coating structures according to variousembodiments. In each case, a single, exemplary nanocomposite 704 isshown encased by the composite protective coating structures forillustration purposes. However, it will be appreciated that theexemplary nanocomposite 704 may in fact be composed of clusters ofnanocomposites, part of a mixed material composite comprising one ormore nanocomposites and other secondary materials, such as a binder,carbon conductive additive, or the like, as discussed above, and soforth.

In the example designs shown in FIGS. 7A-7C, the composite protectivecoating is formed from an outer protective layer 712 (in this case, ametal oxide or metal fluoride shell) and an inner protective layer 720(in this case, a carbon-based material). The use of a metal oxide ormetal fluoride outer protective layer 712 prevents the diffusion ofsolvent molecules and other harmful or reactive molecules, and keepsthem from reaching the surface of the active material of thenanocomposite 704. The use of a carbon-based inner protective layer 720between the nanocomposite 704 and the metal oxide or metal fluorideouter protective layer 712 provides an electrically and ionicallyconductive path for the flow of both electrons and active ions, such asLi in the case of a Li-ion battery. The carbon-based inner protectivelayer 720 also forms an additional barrier against the diffusion ofsolvent molecules and other harmful or reactive molecules, furtherreducing the potential degradation of the underlying active material ofthe nanocomposite 704.

The combination of a mechanically stable outer shell structure that isboth solid and an insoluble ionic conductor, in the form of the metaloxide or metal fluoride outer protective layer 712, for example, and asofter, flexible inner coating in the form of the carbon-based innerprotective layer 720 therefore offers the advantages of both materialsin a single, composite design. Further advantages may be achieved,however, through different structures of the inner protective layer 720,which may be formed in a variety of ways with different properties anddifferent corresponding advantages. The various designs shown in FIGS.7A-7C illustrate several examples.

In the design of FIG. 7A, the inner protective layer 720 is formed as asolid layer, such as an electrically conductive and ionically permeablecarbon coating. This type of dense carbon layer helps to weaken thebonding between the active material of the nanocomposite 704 and theinner surface of the outer protective layer 712, allowing the outerprotective layer 712 to remain expanded and mechanically stable as thenanocompisite 704 contracts inside of it during delithiation. Asdiscussed above, a mechanical stable protective coating such as theouter protective layer 712 provides for the formation of a more stableSEI layer, which in turn provides additional protections against variousdegradation processes. The carbon coating is also generally flexible,and provides some cushion for the expansion of the nanocomposite 704during lithiation, lessening any stress on the outer protective layer712.

However, for some applications, additional expansion capacity may berequired. Accordingly, in the design of FIG. 7B, the inner protectivelayer 720 is formed as a hollow layer, with thin coatings ofcarbonaceous material on the outer surface of the active material 704and the inner surface of the outer protective layer 712, leaving a voidor pore 716 in between. The additional volume of the pore 716 providesfurther space for active material expansion during lithiation, and mayhelp to further alleviate any stress that may be exerted on the outerprotective layer 712 as compared to the denser carbon coating in thedesign of FIG. 7A. These thin coatings on the outer surface of thenanocomposite 704 and the inner surface of the outer protective layer712 still provide the advantages of an intervening electrically andionically conductive coating as in the design of FIG. 7A, but to a morelimited extent under the tradeoff of improved mechanical stressreduction.

The further design of FIG. 7C represents a more general and intermediateapproach, in which the inner protective layer 720 is formed with aseries of smaller pores 716 between the nanocomposite 704 and the innersurface of the outer protective layer 712. It will be appreciated thatthe pores 716 may not be perfectly arranged solely between the activematerial of the nanocomposite 704 and the inner surface of the outerprotective layer 712 as shown for illustration purposes, and mayinstead, at times, infiltrate the active material of the nanocomposite704 or form between any clusters of material making up the nanocomposite704 during battery operation. This arrangement is intended to beincluded in the above description of the series of pores 716 as beingformed “between” the nanocomposite 704 and the inner surface of theouter protective layer 712.

The density of the pores 716 may be tailored to fit a particularapplication (e.g., a particular active material), and adjusted toachieve a desired balance between the amount of carbon in the innerprotective layer 720 (and hence, its electrical and ionic conductivitycharacteristics) and the amount of free space in the inner protectivelayer 720 (and hence, its capacity for expansion during lithiation anddelithiation). For some applications including a silicon activematerial, it has been found that a total pore volume of approximately70% of the volume occupied by the silicon active material core (orgreater) provides sufficient space for expansion during lithiation.

The example designs shown in FIGS. 7D-7F are identical to those shown inFIGS. 7A-7C, respectively, except that they include an additionalconductive coating layer 724 around the outer protective layer 712. Asdiscussed above in relation to the electrical interconnectivity of theconstituent nanocomposites forming the agglomerate structuresillustrated in FIGS. 3A and 3B, such an additional conductive coatinglayer 724 may be used to provide enhanced electrical connectivity amongnanocomposites, which helps form an electrically conductive electrode.The additional conductive coating layer 724 may be formed from carbon ora conductive polymer, for example, and provides a still further barrieragainst the diffusion of solvent molecules and other harmful or reactivemolecules, further reducing the potential degradation of the underlyingactive material in each nanocomposite 704.

FIGS. 8A-8C are cross-sectional views illustrating the effects oflithiation and delithiation on the example composite protective coatingstructures of FIGS. 7D-7F. For simplicity, only the designs of FIGS.7D-7F are shown explicitly, but the effects for FIGS. 7A-7C aresubstantially similar. In addition, the description below relates to asilicon active material and the formation of a Si—Li alloy forillustration purposes, but it will be appreciated that any suitableactive material as discussed above may be alternatively employed.

As shown in FIG. 8A, during initial lithium insertion (e.g., the firstcharge of a Li-ion battery), lithium ions are inserted into the siliconactive material 704 to form a Si—Li alloy 730. During formation, itsouter surface area expands significantly, as described above, displacingand compacting the carbon-based inner protective layer 720.Subsequently, during additional cycling, lithium ions are extracted fromthe Si—Li alloy 730, returning it to its native silicon state 704.Depending on the particular inner protective layer 720 material used andthe volume change experienced, the inner protective layer 720 or aportion thereof may retain its compacted form as shown. Thus, in somecases, the inner protective layer 720 may be compacted, at leastpartially, and a pore or pores 716 similar to those discussed above maybe formed in the intervening space. The formation of such pores 716 inthis manner may be advantageous in some applications (e.g., toaccommodate further volume changes).

Turning to FIG. 8B, for structures of the inner protective layer 720having a large pore design, the intrinsic space provided to accommodateswelling of the active material (such as silicon in the case of an anodematerial for use in Li-ion batteries) may be sufficient to avoid anyinitial insertion effects. In this case, the insertion of lithium ionsinto the silicon active material 704 forms the Si—Li alloy 730, itsouter surface area expands accordingly to fill or partially fill thepore 716, and shrinks back to its original size during delithiation asthe Si—Li-alloy 730 returns to its native silicon state 704.

Turning to FIG. 8C, for intermediate density pore designs, the initiallithium insertion and accompanying expansion of the silicon activematerial 704 to form the Si—Li alloy 730 may cause a displacement of thevarious small pores 716, which typically aggregate together as they arecompacted as shown. Subsequently, during additional cycling, lithiumions are extracted from the Si—Li alloy 730 and it returns to its nativesilicon state 704. Depending on the density and initial arrangement ofthe various pores 716, as well as the volume change experienced, thepores 716 may return to their original configuration and density, or toan altered configuration and density as shown.

Initial lithium insertion, such as that shown in FIGS. 8A and 8C, can beperformed in a variety of ways, including both in-situ (after batterycell assembly) and ex-situ (before battery cell assembly). For example,in-situ lithium insertion into an anode material may involveelectrochemical lithium insertion during operation of the Li-ionbattery, such as insertion of lithium taken from a Li-containing cathodematerial. For ex-situ lithium insertion into an anode material,lithiation may involve gas-phase reactions prior to assembling the anodepowder into the electrode or prior to assembling the electrode into theLi-ion battery cell. Alternatively, ex-situ lithium insertion mayinclude room temperature lithiation of the anode, according to which asoft, stabilized (dry room friendly) lithium powder may be deposited andspread onto a surface of a pre-formed porous anode in order to inducelithiation (primarily upon contact with the electrolyte). If such aprocess is performed in the presence of a solvent, an SEI layer may besimultaneously pre-formed on the anode surface. Lithium foil may be usedinstead of lithium powder in some designs.

It will be appreciated that the different composite protective coatingstructures of FIGS. 7A-7F can be formed in a variety of ways. Severalexample formation methods are described below.

FIG. 9 is a graphical flow diagram from a cross-sectional perspectivedepicting a formation of the example composite protective coatingstructures of FIGS. 7A-7F according to one or more embodiments. In thisexample, formation begins with the provision one or more curved planarnanocomposites 704 in step 910. Again, it will be appreciated that thenanocomposite 704 is shown in the singular for illustration purposesonly, and that the exemplary nanocomposite 704 may in fact be composedof clusters of nanocomposites, part of a mixed material compositecomprising one or more nanocomposites and other secondary materials,such as a binder, carbon conductive additive, or the like, as discussedabove, and so forth.

In step 920, a sacrificial polymer coating 708, for example, is appliedto the nanocomposite 704 as a precursor to the inner protective layer720. The sacrificial polymer coating 708 and nanocomposite 704 are thenfurther coated with the outer protective layer 712 (e.g., a metal oxideor metal fluoride) in step 930, using one or more known methods, such asatomic layer deposition (ALD), chemical vapor deposition (CVD), variouswet chemistry methods, or other methods known in the art.

In step 940, one of the different pore designs of FIGS. 7A-7F isselected, as desired, and formed out of the sacrificial polymer coating708. The particular processes involved depend on the desired structureof the inner protective layer 720 and its associated pores 716 (or lackthereof, as the case may be). In particular, the sacrificial polymercoating 708 can be transformed into the desired structure via heattreatment (e.g., annealing in a gaseous environment), via a hydrothermalprocess, or other techniques as appropriate for the desired structure.Depending on the polymer composition and transformation conditions, thesacrificial polymer may be transformed into: (a) a dense carbonstructure as in the design of FIG. 7A; (b) a thin layer of carbonaceousmaterial covering the nanocomposite 704 and the inner surface of theouter protective layer 712 as in the design of FIG. 7B; or (c) a porouscarbon structure as in the design of FIG. 7C.

For the further designs of FIGS. 7D-7F, additional processing isperformed in step 950 to form the additional conductive polymer orcarbon coating 724, for example, on the surface of the outer protectivelayer 712. For an additional conductive coating 724 formed from carbon,the carbon can be deposited by CVD (e.g., decomposition of hydrocarbongaseous precursors), formation and decomposition of a polymer layer,hydrothermal carbonization, or other techniques. For an additionalconductive coating 724 formed from conductive polymers, the conductivepolymer coating can be deposited via plasma assisted polymerization,microwave assisted polymerization, solution polymerization in asuspension of the nanocomposites to be coated or spray-drying of a jointsuspension of the nanocomposites with the polymer in solution or in theform of polymer nanoparticles.

Returning to the underlying curved planar nanocomposites themselves,regardless of what finishing steps and/or coatings are applied, if any,it will be further appreciated that the nanocomposite structure can beformed in a variety of ways, many of which have already been discussed.

FIG. 10 is a process flow diagram illustrating an example method offorming a curved planar nanocomposite according to one or moreembodiments. In this example, one or more planar substrate backbones areprovided having a curved geometrical structure (step 1010), and eachsubstrate backbone is at least partially encased with a continuous orsubstantially continuous active material film to form one or morecomposite material nanocomposites (step 1020). According to variousembodiments, further processing steps may be performed to obtain thedesired shape, 3-D structure, material composition, etc., as discussedabove. In this example, these further processing steps are representedby the optional, exemplary steps of aggregating the nanocomposites intoa three-dimensional, electrically-interconnected agglomeration (step1030), and/or at least partially filling pores between thenanocomposites in the agglomeration with a solid electrolyte (step1040).

FIGS. 11-13 show example characterizations of particular nanocompositestructures fabricated according to certain embodiments for illustrationpurposes. For these example characterizations, carbon and silicon coatedgraphene electrodes of the type described above were prepared from awater-based slurry containing 15 wt. % poly(acrylic) acid binder. Aftercasting, the electrode was dried in a conventional oven at 70° C. fortwo hours and in a vacuum oven at 70° C. for eight hours. Electrodeswere spot-welded to a 2016-type coin cell and then assembled into halfcells with a lithium foil counter electrode in an Ar filled glovebox(e.g., less than about 1 ppm H₂O, O₂). An electrolyte solutioncontaining 1.0 M LiPF₆ in carbonates was used. Galvanostaticcharge-discharge cycling was performed at between 10 mV and 1 V. TheCoulombic efficiency was calculated by taking a ratio of the capacityafter lithium dealloying to the capacity after lithium alloying. Cyclicvoltammetry in the potential window of 0 to 1.2 V at a rate of 0.014mV·s⁻¹ was performed.

FIGS. 11A-11F present several scanning electron microscopy (SEM)micrographs (FIGS. 11A, 11C, 11E, 11F) and transmission electronmicroscopy (TEM) micrographs (FIGS. 11B, 11D) showing the formation ofone particular example silicon-graphene nanocomposite of the typeillustrated in FIG. 2. SEM images were taken using a 10 kV acceleratingvoltage and a 5-7 mm working distance. TEM studies were performed at anaccelerating voltage of 100 kV.

As shown, silicon nanoparticles were deposited so as to uniformly coatthe smooth graphene sheets (FIGS. 11A-11B), forming a rough continuoussurface (FIGS. 11C-11D). In this example, the silicon has an amorphousstructure with no crystallites being observed in the TEM images. Asshown in FIG. 11E, the carbon layer uniformly coats the silicon surfacereducing surface roughness. As shown in FIG. 11F, the produced compositeparticles approximately retain the original size of the graphene sheets(e.g., about 10-30 μm).

FIGS. 12A-12C illustrate several example characterizations of thegraphene and graphene-based nanocomposite shown in FIGS. 11A-11F byRaman spectroscopy (FIG. 12A), X-ray diffraction (FIG. 12B), and N₂physisorption (FIG. 12C). For the Raman spectroscopy, an exposure timeof five seconds with a 0.6 filter, 400 μm hole, 100 μm slit and 600grating was used. The XRD parameters were as follows: 45 kV acceleratingvoltage, 40 mA current, 0.033° 20-step size, and 120 second record time.N₂ physisorption at −196° C. allowed for the determination of thespecific surface area of the initial and coated materials. Each samplewas degassed in N₂ gas at 100° C. and 300° C. for at least 30 minutesand 8 hours, respectively, prior to the measurements. TheBrunauer-Emmett-Teller method was used to calculate the surface area.

As demonstrated by the Raman spectroscopy of FIG. 12A, the synthesizedgraphene shows nearly the same spectra as commercially availablepurified exfoliated graphite (PEG), indicating that at least someportion of the produced graphene had more than five layers.Characteristic Raman peaks for carbon materials are the disorder-inducedD band at approximately 1350 cm′, the graphitic G band at approximately1580 cm′, the second order G′ band (also known as the 2D band) atapproximately 2700 cm′, and the D″ band (also known as the D+G band) atapproximately 2900 cm′. The large D peak and the small relativeintensity of the 2D peak indicate a high concentration of defects,dangling bonds, and structural disorder present. These defects may serveas nucleation sites for silicon nanoparticle deposition, and allowuniform coating formation. As demonstrated by the Raman spectroscopy ofFIG. 12A, in this particular example, deposition of silicon is on theorder of approximately 520 cm⁻¹. The lack of carbon peaks in thesilicon-coated sample (FIG. 12A) indicates conformal silicon deposition,consistent with the SEM and TEM measurements of FIGS. 11A-11F. Thecarbon deposition is confirmed by the reappearance of D and G peaks. Thesmall shift in the silicon band suggests stresses caused by thedifference in the thermal expansion of silicon and carbon in thisparticular example.

As shown in FIG. 12B, weak graphite peaks of approximately 26° andapproximately 43° appear in the X-ray diffraction (XRD) pattern for thisdesign, but are suppressed after silicon coating. The broad siliconpeaks again indicate an amorphous structure of silicon on the graphenesurface. As evidenced by the formation of diffraction peaks, carbondeposition at higher temperatures may cause crystallization of silicon.

As shown by analysis of N₂ sorption isotherm on the produced graphene inFIG. 12C, a very high specific surface area may be achieved (e.g., about940 m²·g⁻¹ in this example). Micropores may also be present to varyingdegrees. However, silicon and carbon deposition may help eliminate thesmallest pores and reduce the surface area of the produced nanocomposite(e.g., to approximately 5 m²·g⁻¹ in this example).

FIGS. 13A-13C illustrate example electrochemical performance data forsuch a C—Si-graphene electrode. In particular, FIG. 13A shows samplecapacity data (per mass of the composite) and Coulombic Efficiency (CE)data as a function of cycle number, FIG. 13B shows a sample differentialcapacity plot, and FIG. 13C shows sample charge-discharge profiles forselected cycles.

In this particular example, electrochemical performance evaluation ofthe produced nanocomposite was performed in the potential range fromabout 10 mV to about 1 V in 2016-type coin cells with a metallic lithiumfoil counter electrode. As shown in FIG. 13A, the first cycle recordedat the low current density of 140 mA·g⁻¹ showed a high reversibledischarge capacity of approximately 2300 mAh·g⁻¹ (1080 mAh·cc⁻¹). Thisreversible capacity is over six times greater than the theoreticalcapacity of graphite, thus indicating a high degree of lithium alloyingwith silicon. Increasing the current density to 1400 mA·g⁻¹ resulted inthe reduction of the average specific reversible capacity toapproximately 1060 mAh·g⁻¹. Stability of the produced electrodes for 150cycles is particularly noteworthy considering that the lithium insertioncapacity and the resulting silicon expansion were not limited. Anotheradvantage of the produced nanocomposite in this example is its high CE(e.g., about 99%, on average), owing at least in part to the lowcomposite surface area, planar silicon geometry, and good cyclestability.

As shown by the cyclic voltammetry analysis of the potentials at whichlithium (de-) alloying occurs in FIG. 13B, broad lithiation peaks areobserved at 0.08 and 0.22 V vs. Li/Li⁺, consistent with carbon-coatedsilicon. Here, the lithium was extracted from silicon at 0.49 V. Thepeak height slightly increases after the second cycle, indicatingimproved cycling kinetics. Carbon delithiation occurs at potentialslower than silicon and corresponding peaks are not evident in thedifferential capacity curves due to the significantly larger lithiumcapacity of silicon.

The charge/discharge voltage profiles of FIG. 13C show transformationsin the electrode during cycling. The shapes of the profiles are similarto the profiles for other silicon electrodes. With increasing cyclingnumber, the lithium extraction profiles become more horizontal andexhibit slightly smaller overpotential, suggesting a gradual improvementin the discharge kinetics.

FIG. 14 illustrates an example Li-ion battery in which the abovedevices, methods, and other techniques, or combinations thereof, may beapplied according to various embodiments. A cylindrical battery is shownhere for illustration purposes, but other types of arrangements,including prismatic or pouch (laminate-type) batteries, may also be usedas desired. The example Li-ion battery 1 includes a negative anode 2, apositive cathode 3, a separator 4 interposed between the anode 2 and thecathode 3, an electrolyte impregnating the separator 4, a battery case5, and a sealing member 6 sealing the battery case 5. It will beappreciated that the example Li-ion battery 1 may simultaneously embodymultiple aspects of the present invention in various designs.

The preceding description is provided to enable any person skilled inthe art to make or use embodiments of the present invention. It will beappreciated, however, that the present invention is not limited to theparticular formulations, process steps, and materials disclosed herein,as various modifications to these embodiments will be readily apparentto those skilled in the art. That is, the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention, which should only be defined by thefollowing claims and all equivalents.

1. A battery electrode composition, comprising: a composite materialcomprising one or more nanocomposites, wherein the nanocomposites eachcomprise: a planar substrate backbone having a curved geometricalstructure, and an active material forming a continuous or substantiallycontinuous film at least partially encasing the substrate backbone.